This invention relates generally to processes of forming microstructure pattern replicas on a flexible substrate by use of a mold that holds liquid resin on the substrate during curing of the resin, specifically when the microstructure is in the form of a light image carrying surface relief hologram or other light diffraction pattern.
Great Britain published patent application no. 2,027,441 provides a good background of various casting techniques for forming microstructure surface relief patterns on substrates, particularly when the microstructure forms an optical element or is holographically formed. FIG. 9 of this patent publication shows a continuous process for forming such microstructures on a continuous substrate web that is advanced over a portion of the circumference of a rotating drum without relative movement therebetween. The outside circumference of the drum holds a surface relief pattern in the form of the microstructure to be replicated, in the nature of a mold. A liquid casting resin is applied to the rotating drum surface just before it comes into contact with the substrate web. Thus, the casting material is contained between the surface relief pattern mold on the drum and the substrate web where the web and drum are in contact. The resin in this region is hardened by curing, usually by directing actinic radiation, such as ultraviolet light or an electron beam, through the substrate to the resin. When the substrate web is then separated from the drum, it contains hardened resin material on one surface. The exposed surface of the resin material then contains a replica of the microstructure of the master mold on the drum.
Such casting techniques are being applied to the mass replication of holograms for commercial purposes. As is well known, a master hologram is usually made on a photosensitive material, such as a photoresist film, that results in the holographic information being contained in a surface relief pattern. That is, the master hologram surface contains undulations across it that contain holographic information in their shape, depth, pattern and spatial frequency. A hologram pattern usually contains over 10,000 grooves (lines) per inch across its surface, more typically between 30,000 and 40,000 lines per inch. The images recorded on a hologram are reconstructed by illuminating the surface relief pattern. The illuminating light is diffracted and refracted by the surface relief pattern to reconstruct the originally recorded images.
Such a master hologram is physically delicate and cannot be directly used to mass reproduce copies. Therefore, typically, a thin metal film is electrodeposited on the master optical hologram in a manner that it faithfully follows the surface relief pattern. The metal usually used is nickel. Once the optical hologram and metal master are separated, a number of sub-masters are made from it. These submasters are then joined together in order to form a large sheet that can be wrapped around a casting drum. Casting material is then coated onto the substrate web or the drum prior to their surfaces coming into contact. The resin is hardened by curing to complete an initial formation of hologram replicas.
By using a correspondingly wide drum and substrate web, many replicas of the hologram are formed across the width of the web as well as along its length. A large number of replicas of the single hologram are produced by a single revolution of the drum. These individual holograms are then separated from the web material and applied to individual object surfaces by either a hot stamping process or simply by cutting the individual holograms from the web and adhering them to the object surfaces.
It is a primary object of the present invention to provide a casting technique for holograms or other microstructures that improves the process described above in order to make it better adapted for replicating very large numbers of holograms in a continuous process.
It is also an object of the present invention to provide a casting technique that requires only infrequent shut-downs of casting machinery for cleaning and the like.
It is another object of the present invention to provide a method and apparatus for more easily forming multiple hologram masters for attachment to a replicating drum or other continuous surface.
It is yet another object of the present invention to provide a casting technique that provides increased flexibility in how it is used and in the types of products that can be formed therefrom.
It is a further object of the present invention to provide an improved structure of a hologram or other light diffraction grating, and processes of forming such a structure.